Sometimes things are not as they appear. Nowhere is this more true than the Computer Industry. Technology should be very straightforward, but all too often, marketing concerns get thrown into the mix and we end up with specifications with a marketing slant. It has been observed in reviews of the nForce3-150 boards that they often overclock better than VIA K8T800 boards. The nF3 sports a PCI/AGP adjustment in BIOS and it is usually stated that the nF3 should overclock better because of the fixed AGP/PCI bus.

Generally, we have also seen that Intel chipsets with Northwood processors reach incredible overclock levels. Intel has a widely published PCI/AGP lock in 865/875 chipsets, and it is clear that speeds like DDR550 (275x4 FSB) could never be reached without the fixed or locked AGP/PCI speeds. Most peripherals simply can't handle a PCI speed of 45.8 (275/6) or AGP of 91.7 (275/3) when specification is 33/66. Our Radeon 9800 PRO cards can rarely handle anything above about 70-72 on the AGP bus; this is typical of most current AGP cards.

Many tests have demonstrated the ability of Intel chipsets to lock the PCI/AGP bus, but questions are being raised again with the introduction of VIA's new PT880 chipset for the Pentium 4. VIA has been historically the only chipset maker not to utilize a PCI/AGP lock, but in the new PT880, VIA claims that they have implemented an asynchronous PCI/AGP bus. This is another way of saying that the processor and memory bus run at one speed and the PCI/AGP bus is not run at fixed divisors of that speed. This keeps the sensitive PCI and AGP cards in specification and allows a potentially higher overclock of the CPU and memory. Since this is VIA's first AGP/PCI, many are asking whether VIA really pulled this off - it has been such an issue with their chipsets in the recent past.

The most effective way to measure PCI/AGP bus speed is probably oscilloscope testing of bus input frequencies. To see an example of how this is done, you may want to check out tests on the nForce3-150 PCI bus at Lost Circuits. There is also a simpler tool available with the feature of measuring and displaying PCI bus speed, and that is PC Geiger. This device is a card that is mounted in a PCI slot and a digital display that provides all kinds of information - post codes, bus utilization, and PCI speed. The Taiwanese company, IOSS, markets PC Geiger in a joint venture with VICS in Japan. While PC Geiger provides many types of information, we will only be looking at PCI speed for these tests.

PCI Speed and Overclocking: How We Tested
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  • gordon151 - Monday, February 16, 2004 - link

    All the chipsets for the A64 don't have PCI locks, why are you singling out VIA Icewind??? Reply
  • Wonga - Monday, February 16, 2004 - link

    I think it is a bit pathetic how none of the chipsets for the Athlon 64 can manage to get a working PCI lock in them.

    Come on SIS/VIA/nVidia, you can do better than that. How they can do it for the Pentium 4 but screw up on the Athlon 64 is pathetic.
    Reply
  • Icewind - Monday, February 16, 2004 - link

    The first SATA did rely on the PCI bridge but the newer ones have their own dedicated bus to the southbridge so it is not affected by OC. ATA is still attached to the PCI bus if I remember correctly. Reply
  • lebe0024 - Monday, February 16, 2004 - link

    What about harddrive controlers? Does SATA depend on the PCI bus? how about PATA? Reply
  • Icewind - Monday, February 16, 2004 - link

    Cool, no wonder my P4C800-E Deluxe is such a kick ass overclocker. Another reason why not to buy VIA chipset mobo's. Lets hope Nvidia can make a good 939 pin mobo later this year, VIA still leaves a bad taste in my mouth Reply
  • PrinceGaz - Monday, February 16, 2004 - link

    Would it have been possible to raise the FSB to 233 or 234 on those A64 chipsets to see if a 1/7 divider kicks in? I believe the A64 is partially multiplier-unlocked and can have it lowered because of the Cool n Quiet power-saving technology so the CPU itself shouldn't have been a problem.

    If current 8x AGP cards are so sensitive to AGP speed, why not use a slower AGP mode or use an older card which can tolerate a far wider range for some of the tests? I've got an old 2MX somewhere that was quite happy with a 75MHz AGP speed (150FSB on a KT266A chipset, equivalent to 225FSB on current boards) and may well have gone somewhat higher but my PC2100 memory couldn't take any more.

    Interesting article though, surprising about the nForce3 board.
    Reply

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